1. Field of the Invention
The present invention relates to a method of forming a fully adherent, planar photoresist coating film over a preexisting structure. More particularly, the present invention relates to a method for curing a liquid photoresist film such that excess solvent may be driven from the film without causing the film to de-bond and thereby peel from the coated substrate.
2. Description of Related Art
Spin coating is a widely used method to coat a resist solution uniformly over the wafer surface. In this technique, a semiconductor wafer is secured onto a spin chuck and rotated. While the semiconductor wafer is being rotated, a resist solution is supplied to the surface of the semiconductor wafer along its axis of rotation from a nozzle positioned above the wafer. The angular momentum of the spinning wafer forces the resist solution to spread out toward the circumference of the semiconductor wafer. Once sufficient amount of the resist solution is applied to cover the surface of the wafer the flow out of the nozzle is terminated and the semiconductor allowed to continue to rotate while gradually decreasing its rotational speed as the applied resist layer is allowed to dry.
By processing the wafer in this fashion it is also possible to apply a planar coating over the pre-existing surface topology, i.e., where the wafer surface has been patterned and etched. All techniques currently available that enable patterning over a pre-existing structure typically involve the application of a cover layer followed by a time consuming planarization process. The present invention avoids this requirement.
The current techniques also require removal of all or nearly all of the resist solvent component in order to obtain suitable lithographic patterning results, typically by baking the coated wafer at temperatures about or in excess of 100xc2x0 C. However, if the coating covers a pre-existing surface structure the drying process is non-uniform and the resist layer tends to pull away or xe2x80x9cpeel,xe2x80x9d from the surface.
To overcome the problem of resist peeling, Shirakawa, et al., (U.S. Pat. No. 5,629,128) and others teach to add an adhesive aid. The applicant has found that by heating at a lower temperature (55xc2x0 C.) for several hours a significant amount of the solvent component is driven off without causing the resist layer to peel, but enough solvent remains to cause poor lithographic image transfer during patterning resulting in a degraded pattern rendering.
Therefore, it is an object of the present invention to provide a process for applying a planar fully adherent, uniformly thick photoresist layer onto a wafer surface having a pre-existing structure or topology.
Another object of this invention is to provide a method for applying a planar layer of photoresist to a pre-existing surface structure and which will not peel away from that structure.
It is another object to provide a method for minimizing patterning distortion in a photoresist layer that is applied over an existing surface structure and that is subsequently lithographically imaged and developed.
Yet another object of this invention to provide a method for patterning features within a microchannel trench, such as in a microfluidic channel.
The foregoing objects are meant as illustrative of the invention only and not as an exhaustive list. These and other objects will become apparent to those having ordinary skill in these arts as the invention is described in detail.
These and other objects of the invention will be apparent to those skilled in the art based on the teachings herein. The present invention is a condensation method for applying and maintaining a thin layer of liquid solvent on the surface of an applied photoresist layer and thereby exceed the evaporation pressure of the solvent within the resist layer and slowing the loss of solvent from that layer. By controlling the rate at which solvent evaporates from the resist layer it has been found that tendency of the applied resist layer to peel away from the surface structure of the wafer to be minimized or eliminated altogether.